1. Field of the Invention
The present invention is related to a system of coding a moving image, more particularly to the coding of a moving image signal used in a television conference system.
2. Description of the Related Art
In the prior art, to increase the efficiency of the coding of a moving image(motion and time-varying imagery) of a picture, a predictive error prediction coding system, for example, an inter-frame prediction system or a movement compensation prediction system, utilizing a correlation between picture frames, is known. Also, it is known that an orthogonal transformation coding can be applied to a predictive error coding system by utilizing the phenomenon that higher order frequencies have a low energy when a picture. signal is expressed in the form of frequencies.
However, many difficulties are encountered in coding the predictive error signal. Picture signals, and especially moving image signals, are not steady. Among the picture frames for the moving image signals, there are frames representing abrupt motion and frames representing moderate motion. In addition, even a single frame includes a portion representing abrupt motion and a portion representing moderate motion. A simple pattern may be included in a given frame, and the correlation between adjacent pixels in that pattern often cannot be easily established, and thus the local characteristics of the moving image signals are greatly changed. If a single coding method is used for various situations, a high coding efficiency cannot be attained. In other words, if different coding methods employed in correspondence with the picture signal characteristics which are locally changed are not used, a highly efficient coding cannot be attained.
In a picture portion representing a considerable motion, when an intra-frame difference signal is calculated, many coded signals must be transmitted compared with the case where an input picture signal is coded without processing. Even if a total change in one block subjected to orthogonal transformation is small, a specific frequency component, for example, only a DC component, is often changed. When this value is transmitted, a mean value of the block is varied, resulting in tile-like noises, and thus the quality of the displayed picture is degraded.